Systems and methods for fabricating metallic microchannels

ABSTRACT

Embodiments disclosed are systems and methods for fabricating microchannels in metal. In an embodiments, a method includes providing a first metallic plate having a first surface with an elongated slot recessed therein, providing a second metallic plate having a second surface, interfacing the first surface of the first metallic plate with the second surface of the second metallic plate with the second surface covering the elongated slot to form a microchannel between the first metallic plate and the second metallic plate, thermal bonding the first metallic plate to the second metallic plate to form a metallic body having the microchannel extending therethrough, and infiltrating the metallic body with an infiltrant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 63/227,041 filed on Jul. 29, 2021, the disclosure of which is incorporated herein, in its entirety, by this reference.

BACKGROUND

The ability to create high resolution complex structures has enabled additive manufacturing (AM) to revolutionize some microfabrication processes with applications ranging from micro robotics to multilayer printed circuits. Recent efforts have enabled the use of AM in microfluidic applications by using polymer AM to create complex microchannels. Traditionally, microfluidic devices have been fabricated using cleanroom lithography with etching or molding fabrication processes. These processes allow for precise microscale features, but they are only compatible with a few materials and lack the ability to form complex 3D designs. Some microfluidic applications like micro heat exchangers or microcolumns for gas chromatography could benefit from the design flexibility inherent in additive manufacturing but call for high temperatures and high thermal conductivity. Laser powder bed fusion (LPBF) is one method that has been used to form metal microchannels for microfluidic applications.

SUMMARY

Embodiments disclosed herein are systems and methods for fabricating metallic microchannels. In an embodiment, a method for fabricating a microchannel in metal includes providing a first metallic plate having a first surface with an elongated slot recessed therein and providing a second metallic plate having a second surface. The method also includes interfacing the first surface of the first metallic plate with the second surface of the second metallic plate with the second surface covering the elongated slot to form a microchannel between the first metallic plate and the second metallic plate. The method also includes thermal bonding the first metallic plate to the second metallic plate to form a metallic body having the microchannel extending therethrough.

In an embodiment, a microfluidic device is disclosed. The microfluidic device includes a plurality of plates that are thermal bonded together to form a body having an inlet and an outlet, pores of the plurality of plates being at least partially filled with an infiltrant. The microfluidic device also includes a microchannel extending three-dimensionally through the within the metallic structure between the inlet and the outlet, wherein the microchannel has a lateral width or diameter of about 1000 μm or less.

In an embodiment, a microfluidic device is described. The microfluidic device includes a stainless steel metallic structure having an inlet and an outlet. The microfluidic device also includes a microchannel disposed within the metallic structure and extending between the inlet and the outlet. The microchannel has a lateral width or diameter of about 1000 μm or less and extends within the metallic structure (1) at least partially along a first theoretical plane, (2) at least partially along a second theoretical plane angled relative to the first theoretical plane, and (3) at least partially along a third theoretical plane angled relative to the first theoretical plane and the second theoretical plane.

Features from any of the disclosed embodiments may be used in combination with one another, without limitation. In addition, other features and advantages of the present disclosure will become apparent to those of ordinary skill in the art through consideration of the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate several embodiments of the present disclosure, wherein identical reference numerals refer to identical or similar elements or features in different views or embodiments shown in the drawings.

FIG. 1A is a diagram illustrating predicted infiltration of a sample without pressure control showing the potential for either an incompletely filled printed matrix or partially filled channels.

FIG. 1B is a diagram illustrating predicted infiltration of a sample with gravity controlled infiltrant pressures.

FIG. 1C is a diagram illustrating predicted infiltration of a sample with sacrificial powder controlled infiltrant pressure.

FIGS. 2A-2E are various views of a printed part during an infiltration process, according to an embodiment.

FIG. 3 is an image of a milled sample post-infiltration taken from square 3 of FIG. 2D.

FIG. 4 is a flow diagram of a method for fabricating a microchannel in metal, according to an embodiment.

FIG. 5A is a metallic plate including an elongated slot, according to an embodiment.

FIG. 5B is an exploded view of an assembly for sintering and/or infiltrating metallic plates to form a metallic body including a microchannel, according to an embodiment.

FIG. 6A is an isometric view of a metallic structure of a microfluidic device, according to an embodiment.

FIG. 6B is a cross-sectional view of the metallic structure of FIG. 6A.

DETAILED DESCRIPTION

Embodiments disclosed herein include systems and methods for fabricating metallic structures having microchannels extending therethrough. In some embodiments, stainless-steel binder jet three-dimensional (3D) printing and bronze infiltrations are used to create long, compact microchannels in a metallic structure (e.g., metallic block or metallic chip). For example, the binder jet 3D printer may be used to print a plurality of plates, at least some of which have an elongated slot recessed therein on a face or surface of the respective plate. 3D printing allows for high versatility and complexity in design of the microchannels. The plurality of plates are then stacked, according to an embodiment, such that the elongated slots in the plates are covered by an adjacent plate and a microchannel is formed between adjacent plates. The plurality of plates are then thermally bonded (e.g., sintered) and infiltrated, according to an embodiment. For example, the plurality of plates may be infiltrated at a negative pressure, described in greater detail below. In at least some embodiments, the infiltrant fills pores within the plates near or proximate to the microchannels and between plates, sealing adjacent plates together. In this approach, bronze infiltrant must fill the porous material produced by binder jetting without filling the formed microchannels. Thus, infiltration also may be performed in the presence of a sacrificial powder (e.g., sacrificial powder infiltration (SPI)), which prevents overflow of the infiltrant into the microchannels, according to an embodiment. For example, sacrificial powder reservoirs (with pore size of about 60 μm, in some embodiments) may be used to control infiltrant pressure during infiltration. With pressure control, the infiltrant may selectively fill small particles in the printed plates (with pore sizes of about 3 μm or less) while leaving the microchannels (width or diameter of about 500 μm to about 1000 μm) empty. The resulting metallic structure included effective microchannels. The use of plates during fabrication also allows for easy clearing of excess powder during the process. The metallic structures formed according to the systems and methods described herein may be used in systems including microchannels, such as gas chromatography (GC) and/or micro heat exchangers.

Traditionally, microfluidic devices have been fabricated using cleanroom lithography with etching or molding fabrication processes. These processes allow for precise microscale features, but are only compatible with a few materials and lack the ability to form complex 3D designs. Some microfluidic applications like micro heat exchangers or microcolumns for GC could benefit from the design flexibility inherent in the systems and methods described herein.

Embodiments of systems and methods described herein may follow a capillary model. The capillary model of infiltration predicts that infiltrant will fill small pores preferentially over large pores and that pore filling is determined by fill pressure which is also called capillary pressure (P_(c)). P_(c) is defined as the pressure of the infiltrant in a partially filled pore relative to the adjacent gas. P_(c) is negative and governed by the following equation:

$P_{c} = \frac{{- 4}\sigma{{Cos}(\theta)}}{D}$

where σ is the infiltrant surface tension, θ is the contact angle, and D is the diameter of a long cylindrical pore. In a powder, the interstitial pores are non-cylindrical, with P_(c) related to powder particle diameter by:

$P_{c} = {\frac{{- 6}\sigma}{D_{sv}}\frac{\left( {1 - \varepsilon} \right)}{\varepsilon}}$

where ε is the porosity of the powder and D_(sv) is the mean surface volume particle diameter.

The capillary model predicts that during infiltration, whether a pore fills will depend on its fill pressure and the infiltrant pressure. All pores with fill-pressure below (more negative than) the infiltrant pressure will fill, and all pores with fill-pressure above the infiltrant pressure will remain empty. Without external control, pores will fill until all infiltrant is used up. In this case, the final infiltrant pressure is determined by the smallest partially filled pore.

If a part can be infiltrated with the exact amount of infiltrant needed, the porous printed matrix can be filled while the channels can be left empty. However, in practice uncertainty in total pore volume will result sometimes in too much infiltrant, leaving filled channels. Other times the uncertainty would result in too little infiltrant, leaving a partially infiltrated printed matrix. This situation is illustrated in the plot in FIG. 1A, which illustrates infiltration of the printed matrix and microchannels without infiltrant pressure control. The sometimes-filled region (shown in cross hatch) comes from the uncertainty in total pore volume. Infiltrant pressure control can be used to overcome the problem caused by uncertainty in pore volume and is illustrated in FIGS. 1B and 1C. FIG. 1B illustrates gravimetric pressure control, wherein the infiltrant pressure is set above the fill pressure of the porous printed matrix but below the fill pressure of the microchannels. FIG. 1C illustrates sacrificial powder pressure control wherein infiltrant pressure is controlled by the size of the pores in a sacrificial powder reservoir. Uncertainty in total pore volume is still present but this uncertainty impacts only sacrificial powder, resulting in filled printed matrix and completely empty microchannels. Embodiments of systems and methods described herein utilize sacrificial powder pressure control as illustrated in FIG. 1C.

One advantage of using sacrificial pores to control infiltrant pressure, rather than gravity, is that sacrificial pores can more practically be used to keep small structures (such as microchannels) from filling. For example, a 250 μm channel is predicted to fill with infiltrant unless the infiltrant pressure is below −16 kPa, which would require a height of about 21 cm using gravimetric pressure control (calculated using ΔP=ρgh, where ρ is the infiltrant density and g is the gravitational constant). This large height would make it more difficult to implement due to issues including the large furnace size and long infiltrant transport distances that would be required. However, a sacrificial powder reservoir with an effective pore diameter smaller than about 250 μm (powder diameter <500 μm) will achieve these infiltrant pressures.

In Example 2, described below, it is valuable to understand how the capillary pressure and fill height (maximum height of filled pores) compare for different sized pores. The difference in capillary pressure between two different diameter pores D₁ and D₂ can be derived using eq. 2, resulting in:

${\Delta P_{c}} = {4\sigma{Cos}(\theta)\left( {\frac{1}{D_{2}} - \frac{1}{D_{1}}} \right)}$

The infiltrant pressure P_(i) also varies with height according to ΔP_(i)=ρgh. By modifying this equation, the expected difference in fill height between two pores can be found:

${\Delta h} = {\frac{4\sigma{{Cos}(\theta)}}{\rho g}\left( {\frac{1}{D_{2}} - \frac{1}{D_{1}}} \right)}$

At pressure equilibrium, the capillary model predicted a uniform fill-height for channels of the same diameter. It also predicts the quantitative difference in fill-height between channels of different diameter.

In some embodiments, it was discovered that, through capillary action, a vertical cylindrical capillary will fill to a certain height H based on its radius r, the surface tension T, the contact angle θ, the density ρ of the fluid, and the acceleration of gravity g:

$H = \frac{2T{Cos}(\theta)}{\rho gr}$

This equation was applied to control infiltrant filling of variously sized pores. Capillary action caused pores with smaller radii to be filled before larger ones. By introducing a sacrificial pore size larger than the pore size of the stainless steel matrix and smaller than the channel diameter, the infiltrant flow can be controlled enough to seal the unwanted pores while keeping the channels clear. Sacrificial pores may be incorporated by placing a large grained powder around the printed channel.

In at least one, some, or all embodiments described herein, methods and systems are provided for sealing the porosity in binder jet printed parts with sub-millimeter scale internal structures. According to an embodiment, after printing and thermally bonding (e.g., sintering) a part, but prior to bronze infiltration, a reservoir of sacrificial powder is placed in contact with the part. While a sacrificial powder is referenced herein, in some embodiments other sacrificial agents having properties other than powder may be utilized. For example, the sacrificial agent may include a porous material that the infiltrant wets to during processing (e.g., heating). Accordingly, in some embodiments, a sacrificial porous metallic sponge having predetermined pore sizes (described below) may be used in place of the sacrificial powder described herein. In some embodiments, a sacrificial porous ceramic material configured such that the infiltrant wets to the sacrificial porous ceramic material may be used in place of the sacrificial powder described herein. In some embodiments, the sacrificial powder may be replaced with a sacrificial agent including a block (e.g., metallic block) having holes drilled or cut therein effect to provide constant pore size through the block. In some embodiments, the sacrificial powder may be replaced with a sacrificial agent including a bundle of wires (e.g., metallic wires). Another possible sacrificial powder or bead based sacrificial material may include ceramic particles and/or beads, or a reservoir of ceramic particles and/or beads that are used as is or activated for metal infiltrant wetting prior to infiltration. The ceramic materials may include one or more of silica, alumina, tungsten carbide, silicon carbide, chromium oxide, zirconium oxide, or combinations thereof. The activation may include coating or chemical surface modification (e.g., reduction, oxidation, or small molecule bonding) to create a surface that will wet metals. Processes that could also include chemical modification or functionalization, painting and firing, or metal plating (electroless or electroplating) or a combination thereof. In one embodiment, the ceramic surface could coated with a metal paint, like a molybdenum based paint, and fired prior to infiltration.

Turning to FIGS. 2A-2E, an example of sacrificial powder infiltration process is provided. In FIG. 2A, a cylinder 200 is provided, having been 3D printed according to any of the printing processes described herein. The cylinder 200 includes an interior region 210 and a plurality of microchannels 220 disposed within an inner cylinder inside the cylinder 200. The cylinder 200 may be thermally bonded, as shown in FIG. 2B. Thermal bonding may include one or more of sintering, brazing, soldering, and/or combinations thereof. After thermal bonding, a sacrificial powder 215 is inserted into the interior region 210 of the cylinder 200, as shown in the cross-sectional view of FIG. 2C. The sacrificial powder 215 has pore dimensions larger than the pores in the porous printed matrix of the cylinder 200, but smaller than the microchannels 220.

In some embodiments, the sacrificial powder has a particle diameter of about 25 μm to about 250 μm, about 25 μm to about 125 μm, about 125 μm to about 225 μm, about 25 μm to about 50 μm, about 50 μm to about 100 μm, μm 100 to about 150 μm, about 150 μm to about 200 μm, about 200 μm to about 250 μm, about 25 μm to about 35 μm, about 70 μm to about 90 μm, about 175 μm to about 225 μm, about 30 μm, about 80 μm, or about 200 μm. In some embodiments, the sacrificial powder may form pores having a diameter of about 10 μm to about 150 μm, about 10 μm to about 50 μm, about 50 μm to about 100 μm, about 100 μm to about 150 μm, about 10 μm to about 30 μm, about 30 μm to about 50 μm, about 50 μm to about 70 μm, about 70 μm to about 90 μm, about 90 μm to about 110 μm, about 110 μm to about 130 μm, about 130 μm to about 150 μm, about 20 μm, about 50 μm, or about 120 μm.

In some embodiments, the printed matrix has a particle diameter or other lateral dimension of about 2 μm to about 60 μm, about 2 μm to about 20 μm, about 20 μm to about 40 μm, about 40 μm to about 60 μm, about 2 μm to about 10 μm, μm 5 to about 15 μm, about 10 μm to about 20 μm, about 20 μm to about 55 μm, about 30 μm to about 40 μm, about 40 μm to about 50 μm, about 45 μm to about 60 μm about 50 μm to about 60 μm, about 5 μm, about 10 μm, or about 50 μm. In some embodiments, the printed matrix may include pores having a diameter or other lateral dimension of about 1 μm to about 25 μm, about 1 μm to about 8 μm, about 8 μm to about 16 μm, about 16 μm to about 24 μm, about 1 μm to about 5 μm, about 5 μm to about 10 μm, about 10 μm to about 15 μm, about 15 μm to about 20 μm, about 20 μm to about 25 μm, about 2 μm to about 4 μm, about 1 μm to about 3 μm, about 14 μm to about 16 μm, about 2 μm, about 3 μm, or about 15 μm.

In some embodiments, the sacrificial powder includes a particle diameter or other lateral dimension of about 80 μm and a pore diameter of about 50 μm, while the printed matrix includes a particle diameter of about 10 μm and a pore diameter of about 3 μm. In some embodiments, the sacrificial powder includes a particle diameter of about 200 μm and a pore diameter of about 120 μm, while the printed matrix includes a particle diameter of about 50 μm and a pore diameter of about 15 μm. In some embodiments, the sacrificial powder includes a particle diameter of about 30 μm and a pore diameter of about 20 μm, while the printed matrix includes a particle diameter of about 5 μm and a pore diameter of about 2 μm.

Accordingly, in some embodiments, the sacrificial powder has a particle diameter or other lateral dimension greater than the pore diameter of the sacrificial powder, such as sacrificial powder particle diameter about 1.1 times, about 1.2 times, about 1.3 times, about 1.4 times, about 1.5 times, about 1.6 times, about 1.7 times, about 1.8 times, about 1.9 times, or about 2 times greater than the pore diameter of the sacrificial powder. In some embodiments, the printed matrix has a particle diameter greater than the pore diameter of the printed matrix, such as a printed matrix particle diameter about 2 times, about 2.5 times, about 3 times, about 3.5 times, or about 4 times greater than the pore diameter of the printed matrix. In some embodiments, the pore diameter or other lateral dimension of the sacrificial powder is greater than the pore diameter of the printed matrix, such as the pore diameter of the sacrificial powder being about 5 times to about 25 times, about 5 times to about 10 times, about 10 times to about 15 times, about 15 times to about 20 times, or about 20 times to about 25 times greater than the pore diameter of the printed matrix. In some embodiments, width or diameter of the microchannel is greater than the pore diameter of the sacrificial powder, such as the width or diameter of the microchannel being at least about 5 times, at least about 10 times, about 5 times to about 10 times, about 10 times to about 15 times, about 5 times to about 7 times, about 7 times to about 9 times, about 9 times to about 11 times, about 11 times to about 13 times, or about 13 times to about 15 times greater than the pore diameter of the sacrificial powder.

The cylinder 200 (including the sacrificial powder 215) is then positioned in a container 250 (e.g., crucible) proximate the infiltrant 260, as shown in the cross-sectional view of FIG. 2D. During infiltration processing, after the infiltrant 260 fills the porous printed matrix, excess infiltrant then fills the sacrificial powder 215 instead of the microchannels 220. In some embodiments, local pressure also can be controlled, thereby reducing the impact of pressure variations across the metallic structure.

FIG. 3 is an image of a milled sample post-infiltration taken from square 3 of FIG. 2D. FIG. 3 shows a fully dense printed matrix 270 that is infiltrated with bronze such that the fully dense printed matrix 270 is substantially devoid of pores (e.g., there are substantially no pores in the fully dense printed matrix 270 or pores are substantially absent from the fully dense printed matrix 270), semi-porous sacrificial powder 280 in a solid form after infiltration processing, pores 290 present in the semi-porous sacrificial powder 280 in the solid form, and an unfilled 700 μm microchannel 220 defined by the fully dense printed matrix 270.

FIG. 4 is a flowchart of a method 400 of manufacturing or fabricating a microfluidic device, according to an embodiment. FIG. 5A and FIG. 5B show an assembly 500 in various acts or steps of the method 400. Each of FIGS. 4, 5A, and 5B are referred to below. In an embodiment, a method 400 for fabricating a microchannel in metal includes an act 410 of providing a first metallic plate 505 a having a first surface 506 a with an elongated slot 520 a recessed therein. The method 400 also includes an act 420 of providing a second metallic plate 505 b having a second surface 507 b. The method 400 also includes an act of interfacing the first surface 506 of the first metallic plate 505 a with the second surface 507 b of the second metallic plate 505 b with the second surface 507 b covering the elongated slot 520 a to form a microchannel between the first metallic plate 505 a and the second metallic plate 505 b. The method 400 also includes an act 430 of interfacing (e.g., stacking) the first surface of the first metallic plate with the second surface of the second metallic plate with the second surface effective to cover the elongated slot to form a microchannel between the first metallic plate and the second metallic plate. The method 400 also may include an act 440 of thermally bonding the first metallic plate 505 a to the second metallic plate 505 b to form a metallic body having the microchannel extending therethrough. Thermal bonding may include one or more of sintering, brazing, soldering, and/or combinations thereof. The method 400 also includes an act 450 of infiltrating the metallic body with an infiltrant. Acts of the method 400 are for illustrative purposes. For example, the acts of the method 400 may be performed in different orders, split into multiple acts, modified, supplemented, or combined.

In some embodiments, the acts 410 and 420 of providing the first metallic plate 505 a and the second metallic plate 505 b may include 3D printing the first metallic plate 505 a and the second metallic plate 505 b. The metallic plate 505 may be 3D printed with a metallic material according to any of the 3D printing processes provided herein. In some embodiments, the metallic plate 505 is binder jet 3D printed with a stainless steel material. In some embodiments, metallic plates 505 may be a metal, an alloy, or a composite thereof. In some embodiments, the materials of the metallic plates 505 and the infiltrant may include any materials (e.g., metallic materials) wherein the infiltrant has a lower melting point than the metallic plates 505. In some embodiments, after printing and curing the metallic plate 505, loose powder may be removed from elongated slot using brushes and compressed air.

FIG. 5A shows an example of a metallic plate 505 provided by 3D printing. The metallic plate 505 may include any of the multiple plates (e.g., the first metallic plate 505 a, the second metallic plate 505 b, the third metallic plate 505 c, or the fourth metallic plate 505 d) stacked on one another to form the metallic body. The metallic plate 505 is printed or otherwise formed to include an elongated channel recessed into a surface 506 of the metallic plate 505. In some embodiments, the elongated slot 420 is at least about 10 mm, at least about 15 mm, at least about 20 mm, at least about 25 mm, at least about 30 mm, at least about 35 mm, at least about 40 mm, at least about 45 mm, or at least about 50 mm in length in the metallic plate 505. The elongated slot 420 may have a maximum width of about 1000 μm or less, less than about 900 μm, less than about 800 μm, less than about 700 μm, less than about 600 μm, less than about 500 μm, less than about 450 μm, less than about 400 μm, less than about 350 μm, less than about 300 μm, less than about 250 μm, less than about 200 μm, less than about 150 μm, less than about 100 μm, less than about 75 μm, less than about 50 μm, less than about 25 μm, less than about 15 μm, about 10 μm to about 1000 μm, about 10 μm to about 100 μm, about 100 μm to about 250 μm, about 250 μm to about 750 μm, about 500 μm to about 1000 μm, about 250 μm to about 500 μm, about 500 μm to about 750 μm, about 750 μm to about 1000 μm, about 300 μm to about 400 μm, about 400 μm to about 500 μm, about 500 μm to about 600 μm, about 600 μm to about 700 μm, about 700 μm to about 800 μm, about 800 μm to about 900 μm, or about 900 μm to about 1000 μm. In some embodiments, the elongated slot may include a non-circular or non-semi-circular shape, and may include cross-sectional width of about 10 μm to about 50 μm.

The elongated slot 520 in the surface 506 may be generally semi-circular (e.g., half-circular) or polygonal (e.g., triangular, square, rectangular) in cross-sectional shape. The metallic plate 505 also may be formed (e.g., printed) to include a second or additional surface (not shown in FIG. 5A) opposite to the surface 506. The second surface of the metallic plate 505 may be substantially planar, smooth, or flat (e.g., a slot is absent from the second surface of the metallic plate 505). In some embodiments, however, the second surface 505 may include an elongated slot disposed on the second surface complementary to an elongated slot 520 on the first surface 506 of an additional metallic plate 505 on which the metallic plate 505 is stacked.

The metallic plate 505 also may include (e.g., define) an inlet 521 in fluid communication with the elongated slot 520 when another plate is stacked on the metallic plate 505. In some embodiments, the inlet 521 extends through the metallic plate 505 from the elongated slot 520 to the second surface (e.g., the second surface at least partially defines the inlet). The inlet 521 may be spaced from an edge of the metallic plate 505 and positioned to align with a terminating end 522 of an elongated slot 520 of an adjacent additional metallic plate 505 on which the metallic plate is stacked. In some embodiments, the inlet 521 is positioned at a terminating end of the elongated slot 520. In some embodiments, the inlet 521 is positioned at an edge of the metallic plate 505, and the inlet 521 may serve as the inlet for the entire microchannel of the final metallic structure. In some embodiments, the elongated slot 520 extends to an edge of the metallic plate 505 to serve as an outlet for the entire microchannel of the final metallic structure.

The elongated slot 520 may extend two-dimensionally on the metallic plate 505 from the inlet 521 to the opposing terminating end. For example, on the metallic plate 505 shown in FIG. 5A, the elongated slot 520 extends in a first dimension between the top and bottom of the metallic plate 520 as viewed in FIG. 5A and also extends in a second dimension between the left and right of the metallic plate 520 as viewed in FIG. 5A. Said another way, the elongated slot 520 may extend at least bi-directionally on a theoretical grid of the surface 506 of the metallic plate.

The metallic plate 505 also may include one or more alignment markers 510 or features. The one or more alignment markers 510 may be positioned to align with one or more alignment markers of additional adjacent metallic plates 505 on which the metallic plate 505 is stacked or are stacked on the metallic plate 505. The alignment marker 510 may include a protrusion and/or a recess in the surface 506 (and/or the second surface) of the metallic plate. For example, the metallic plate 505 may include a protruding alignment marker 510 positioned to align with a recessed alignment marker in the surface of an adjacent second metallic plate 505. The metallic plate 505 also may include a recessed alignment marker 510 positioned to aligned with a protruding alignment marker in the surface of an adjacent second metallic plate 505.

In some embodiments, the method 400 also may include disposing a metallic lid plate over the first metallic plate and the second metallic plate with the second metallic plate being positioned between the first metallic plate and the second metallic plate before thermally bonding and infiltrating the metallic plates. For example, FIG. 5B shows an exploded view of an assembly including the metallic plates 505 a-d and a metallic lid plate 525. The metallic lid plate 525 includes or defines an opening 526 that is in fluid communication with elongated slot of each of the metallic plates 505 a-d when the metallic lid plate is stacked on the metallic plates 505 a-d. For example, when (1) the first surface 506 a of the first metallic plate 505 a is interfacing with the second surface 507 b of the second metallic plate 505 b with the second surface 507 b and (2) the metallic lid plate 525 is disposed over the first metallic plate 505 a and the second metallic plate 505 b, the opening 526 on the metallic lid plate 525 is in fluid communication with the elongated slot 520 a in the first surface 506 a of the first metallic plate 505 a via at least the inlet 521 (not visible in FIG. 4B) in the second metallic plate 505 b positioned proximate to a terminating end region of the elongated slot 520 b. In these and other embodiments, the act 440 of thermally bonding the first metallic plate 505 a to the second plate 505 b may include sintering the first metallic plate 505 a, the second metallic plate 505 b, and the metallic lid plate 525 together to form the metallic body having the microchannel extending therethrough with the opening 526 on the metallic lid plate 525 in fluid communication with the microchannel. In some embodiments, the act 440 of thermally bonding may include one or more of sintering, brazing, soldering, and/or combinations thereof.

In some embodiments, the method 400 also may include an act of disposing an additional metallic plate over the first metallic plate and the second metallic plate such that the additional metallic plate is between the second metallic plate and the metallic lid plate. For example, FIG. 5B shows an exploded view of an assembly including four metallic plates 505 a-d. Each of the metallic plates 505 a-d may include an elongated slot 520 a-d. When (1) the first surface 506 a of the first metallic plate 505 a is interfacing with the second surface 507 b of the second metallic plate 505 b with the second surface 507 b, and (2) the metallic lid plate 525 is disposed over the first metallic plate 505 a and the second metallic plate 505 b with the additional metallic plate(s) 505 c-d positioned between the metallic lid plate 525 and the second metallic plate 505 b, the additional elongated slot(s) 520 c-d are in fluid communication with the opening 526 in the metallic lid plate 525 and also the elongated slot 520 a of the first metallic plate 505 a, according to an embodiment. In these and other embodiments, the act 440 of thermally bonding the first metallic plate 505 a to the second plate 505 b may include sintering the first metallic plate 505 a, the second metallic plate 505 b, the additional metallic plate(s) 505 c-d, and the metallic lid plate 525 together to form the metallic body having the microchannel including at least the elongated slot 520 a and the additional elongated slot(s) 520 b-d extending therethrough with the opening 526 on the metallic lid plate 525 in fluid communication with the microchannel. When the metallic plates 505 a-d are sintered together and a microchannel is formed from the elongated slots 520 a-d in the metallic plates 505 a-d, the formed microchannel extends three-dimensionally within the metallic body formed from the sintered metallic plates 505 a-d. For example, the microchannel may (1) extend at least partially between a first side surface region of the metallic body and a second side surface region of the metallic body opposite to the first side surface region, (2) extend at least partially between a third side surface region of the metallic body extending at least partially between the first side surface region and the second side surface region and a fourth side surface region opposite to the third side surface region, and (3) extend at least partially between an upper surface region (e.g., a region that includes metallic plate 505 d) of the metallic body and a bottom surface region (e.g., a region that includes metallic plate 505 a) of the metallic body opposite to the upper surface region.

In some embodiments, the act 440 of thermally bonding the first metallic plate to the second metallic plate to form a metallic body having the microchannel extending therethrough may include sintering the first metallic plate 505 a to the second metallic plate 505 b (and any additional metallic plates 505 c-d and/or the metallic lid plate 525) at a predetermined temperature for a predetermined time in a selected atmosphere to form the metallic body having the microchannel extending therethrough. In some embodiments, during sintering and infiltration, samples (e.g., metallic plates) are inserted into the heating zone and a transfer arm is sealed with PVC tubing using hose clamps to minimize oxygen diffusion into furnace. After sintering or infiltration, samples may be rapidly cooled by unsealing the transfer arm and pulling them out to the cooling zone.

In some embodiments, the metallic plates 505 a-d and the metallic lid plate 525 may be thermally bonded (e.g., sintered, brazed, and/or soldered) at a predetermined temperature of at least about 600° C., at least about 700° C., at least about 800° C., at least about 900° C., at least about 1000° C., about 600° C. to about 1000° C., about 600° C. to about 800° C., about 800° C. to about 1000° C., about 600° C. to about 700° C., about 700° C. to about 800° C., about 800° C. to about 900° C., or about 900° C. to about 1000° C. The temperature may be increased from room temperature to the predetermined temperature at a rate of about 10° C./min to about 15° C./min, about 12° C./min to about 14° C./min, about 10° C./min, about 11° C./min, about 12° C./min, about 13° C./min, about 14° C./min, or about 15° C./min. The act 440 of thermally bonding the first metallic plate to the second metallic plate (and any additional metallic plates) forms a metallurgical bond between the metal plates. In some embodiments, the method 400 may include holding the metallic plates at the predetermined temperature for a predetermined amount of time that may include at least about 20 minutes, at least about 30 minutes, at least about 45 minutes, at least about one hour, at least about 1.5 hours, or at least about 2 hours. Holding the metallic plates at the predetermined temperature for the predetermined amount of time may allow for binder burnout.

In some embodiments, the act 440 of thermally bonding the first metallic plate to the second metallic plate to form a metallic body having the microchannel extending therethrough may include sintering the first metallic plate 505 a to the second metallic plate 505 b (and any additional metallic plates 505 c-d and/or the metallic lid plate 525) at the predetermined temperature for the predetermined time, and then at a second predetermined temperature for a second predetermined amount of time. The second predetermined temperature may be at least about 800° C., at least about 900° C., at least about 1000° C., at least about 1100° C., at least about 1200° C., 800° C. to about 1400° C., about 800° C. to about 1000° C., about 1000° C. to about 1200° C., about 1200° C. to about 1400° C., about 800° C. to about 900° C., about 900° C. to about 1000° C., about 1000° C. to about 1100° C., about 1050° C. to about 1150° C., about 1100° C. to about 1200° C., about 1200° C. to about 1300° C., or about 1300° C. to about 1400° C. The temperature may be increased to the second predetermined temperature at a rate of about 3° C./min to about 10° C./min, about 4° C./min to about 8° C./min, about 4° C./min, about 5° C./min, about 6° C./min, about 7° C./min, about 8° C./min, or about 9° C./min.

In some embodiments, the predetermined temperature (and/or the second predetermined temperature) for the act 440 of thermally bonding is less than the melting temperature of the metallic plate, such as about 0.6 to about 0.95, about 0.6 to about 0.7, about 0.7 to about 0.8, about 0.75 to about 0.85, about 0.8 to about 0.9, about 0.85 to about 0.95, about 0.7, about 0.75, about 0.8, about 0.85, about 0.9, or about 0.95 of the absolute melting temperature of the material of the metallic plate.

In some embodiments, the method 400 may include holding the metallic plates at the second predetermined temperature for the second predetermined amount of time that may include at least about 5 minutes, at least about 10 minutes, at least about 15 minutes, at least about 20 minutes, at least about 30 minutes, or at least about 45 minutes. Holding the metallic plates at the predetermined temperature for the predetermined amount of time may allow for binder burnout.

In some embodiments, the act 440 of thermally bonding the first metallic plate to the second metallic plate to form a metallic body having the microchannel extending therethrough may include sintering the first metallic plate 505 a to the second metallic plate 505 b (and any additional metallic plates 505 c-d and/or the metallic lid plate 525) in a preselected atmosphere. The preselected atmosphere may include at least one (e.g., both) of hydrogen and argon. In some embodiments, the during the final selected minutes of the predetermined amount of time and/or the second predetermined amount of time, hydrogen flow may be stopped and argon flow may be increased to a predetermined rate to flush hydrogen from the tube. The predetermine rate of flow of the argon may be about 1000 SCCM to about 2000 SCCM, about 1200 SCCM to about 1600 SCCM, about 1100 SCCM to about 1300 SCCM, about 1300 SCCM to about 1500 SCCM, or about 1500 SCCM to about 1700 SCCM.

The method 400 may then include rapidly cooling the metallic plates to a cooling zone of about 100° C. to about 300° C., about 150° C. to about 250° C. or about 200° C. The method 400 may further include leaving the metallic plates in the cooling zone, still under the flow of argon, for at least about 10 minutes, at least about 15 minutes, at least about 20 minutes, or at least about 30 minutes.

In some embodiments, the method 400 further comprises disposing a weight on the metallic plates 505 a-d and/or the metallic lid plate 525 before sintering the metallic plates together. For example, a weight of at least about 50 g, at least about 100 g, at least about 150 g, at least about 200 g, at least about 250 g, about 100 g to about 250 g, about 50 g to about 100 g, about 100 g to about 150 g, about 150 g to about 200 g, or about 200 g to about 250 g.

In some embodiments, the act 440 of thermally bonding the first metallic plate to the second metallic plate (and/or any additional metallic plates) may include an act of high-temperature sintering the metallic plates to one another, and the act 450 of infiltrating the metallic body with the infiltrant may be absent from the method. In some embodiments, the act 440 includes thermally bonding the first metallic plate to the second metallic plate at a predetermined temperature of at least about 1250° C., at least about 1300° C., at least about 1350° C., at least about 1400° C., at least about 1450° C., at least about 1500° C., about 1250° C. to about 1500° C., about 1250° C. to about 1350° C., or about 1350° C. to about 1450° C.

In some embodiments, the act 450 of infiltrating the metallic body with an infiltrant includes infiltrating the metallic body with the infiltrant in the presence of a sacrificial powder. The infiltrant may include a bronze powder, a copper powder, a zinc powder, tin and/or combinations or alloys thereof. The material of the metallic plates may include a stainless steel powder, a titanium alloy powder, a ceramic powder, or combinations thereof. The material of the sacrificial powder may include porous metals such as stainless steel, copper, and/or titanium, and/or porous ceramics. For example, in some embodiments, the metallic plate may be formed (e.g., printed) from a stainless steel material (e.g. powder), and the infiltrant may include a bronze infiltrant. In some embodiments, the metallic plate may be formed from a stainless steel material, and the infiltrant may include a copper infiltrant. In some embodiments, the metallic plate may be formed from a titanium alloy material, and the infiltrant may include a copper infiltrant. In some embodiments, the plate may be formed from a ceramic powder, and the infiltrant may include any material that wets to the ceramic powder. In some embodiments, the infiltrant may be a solid rather than a powder.

In some embodiments, infiltrating the metallic body with the infiltrant in the presence of a sacrificial powder includes infiltrating the metallic body with the infiltrant in the presence of a sacrificial powder at a predetermined time and a predetermined temperature to melt at least the infiltrant. The predetermined temperature of infiltration may be at least about 800° C., at least about 900° C., at least about 1000° C., at least about 1100° C., at least about 1200° C., 800° C. to about 1400° C., about 800° C. to about 1000° C., about 1000° C. to about 1200° C., about 1200° C. to about 1400° C., about 800° C. to about 900° C., about 900° C. to about 1000° C., about 1000° C. to about 1100° C., about 1050° C. to about 1150° C., about 1100° C. to about 1200° C., about 1200° C. to about 1300° C., or about 1300° C. to about 1400° C. The predetermined time of infiltration may be at least about 2 hours, at least about 3 hours, at least about 4 hours, at least about 5 hours, at least about 6 hours, at least about 7 hours, about 2 hours to about 8 hours, about 2 hours to about 5 hours, about 5 hours to about 8 hours, about 2 hours to about 4 hours, about 4 hours to about 6 hours, about 6 hours to about 8 hours, about 2 hours to about 3 hours, about 3 hours to about 4 hours, about 4 hours to about 5 hours, about 5 hours to about 6 hours, about 6 hours to about 7 hours, or about 7 hours to about 8 hours.

In some embodiments, the act 450 of infiltrating the metallic body with an infiltrant includes may include infiltrating the metallic body with the infiltrant in a preselected atmosphere. The preselected atmosphere may include at least one (e.g., both) of hydrogen and argon. In some embodiments, atmospheres during infiltration may include one or more of argon mixed with hydrogen, a vacuum (e.g., negative pressure) atmosphere, one or more inert gasses mixed with hydrogen or another reducing gas, and/or a purely hydrogen environment. For example, the method may include first flowing argon into the furnace to flush air from the tube, and then transitioning the metallic body to the heating zone in the presence of a second flow argon and hydrogen. The first flow of argon may about may be about 1000 SCCM to about 2000 SCCM, about 1200 SCCM to about 1600 SCCM, about 1100 SCCM to about 1300 SCCM, about 1300 SCCM to about 1500 SCCM, or about 1500 SCCM to about 1700 SCCM. The second flow of argon may be about may be about 400 SCCM to about 1000 SCCM, about 400 SCCM to about 700 SCCM, about 700 SCCM to about 1000 SCCM, about 600 SCCM to about 800 SCCM, or about 650 SCCM to about 750 SCCM. The flow of oxygen may be at least about 100 SCCM, at least about 200 SCCM, at least about 300 SCCM, about 100 SCCM to about 300 SCCM, or about 150 SCCM to about 250 SCCM.

The method 400 may then include rapidly cooling the metallic body as infiltrated to a cooling zone of about 100° C. to about 300° C., about 150° C. to about 250° C. or about 200° C. The method 400 may further include leaving the metallic plates in the cooling zone, still under the flow of argon, for at least about 10 minutes, at least about 15 minutes, at least about 20 minutes, or at least about 30 minutes.

In some embodiments, the act 450 of infiltrating the metallic body with an infiltrant includes infiltrating the metallic body with the infiltrant in the presence of a sacrificial powder at a predetermine pressure of about −1 kPa or less. Infiltration at the negative pressure keeps the microchannel free of infiltrant during the act of infiltrating the metallic body (e.g., the infiltrant is substantially absent from the microchannel and/or the microchannel is substantially devoid of the infiltrant). In some embodiments, the predetermined pressure during infiltration is about −1 kPa to about −100 kPa, about −1 kPa to about −50 kPa, about −50 kPa to about −100 kPa, about −1 kPa to about −25 kPa, about −25 kPa to about −50 kPa, about −50 kPa to about −75 kPa, or about −75 kPa to about −100 kPa. In some embodiments, the predetermined pressure may be correlated or correspond to a width of the microchannel in the metallic body being infiltrated. For example, if the width or diameter of elongated slot or microchannel is less than 1000 μm, then the pressure will be less than about −1 kPa. If the width or diameter of elongated slot or microchannel is about 50 μm, the pressure will be less than about −50 kPa. In some embodiments, the predetermined pressure is significantly lower than a corresponding width of the microchannel in the metallic body being infiltrated. For example, a pressure of −40 kPa may be used to keep a 1000 μm free of infiltrant.

In some embodiments, the method 400 may include, before infiltrating the body with the infiltrant in the presence of a sacrificial powder, disposing the sacrificial powder adjacent the metallic body and disposing the infiltrant with the sacrificial powder between the infiltrant and the metallic body. For example, FIG. 5B shows a support 530 that may include a region 550 that holds the metallic plates 505 a-d and the sacrificial powder. When the metallic plates are positioned in the region 550, a compartment 555 may be formed between the metallic plates 505 a-d and a divider 560, and the sacrificial powder may be disposed in this compartment. The support 530 also may include a basin 540 for holding the infiltrant, with the divider 560 being disposed between the compartment 550 for sacrificial powder and the basin 540 for the infiltrant.

The act of infiltrating the metallic body in the presence of the sacrificial may form a metallic structure that includes a first region and a second region. For example, FIG. 6B is a cross-sectional view of a microfluidic device 600 formed according to one or more embodiments of the method 400. The microfluidic device 600 includes a first region 670 at least partially (e.g., entirely) defines the microchannel 620 and is substantially devoid of pores 690. The microfluidic device 600 also includes a second region 680 that includes at least a portion of the solidified sacrificial powder and that is spaced from the microchannel 620 by the first region 670. The second region 680 typically includes one or more pores 690.

The resulting microchannel formed in the metallic structure (e.g., microfluidic device may have a maximum width of about 1000 μm or less, less than about 900 μm, less than about 800 μm, less than about 700 μm, less than about 600 μm, less than about 500 μm, less than about 450 μm, less than about 400 μm, less than about 350 μm, less than about 300 μm, less than about 250 μm, less than about 200 μm, less than about 150 μm, less than about 100 μm, less than about 75 μm, less than about 50 μm, less than about 25 μm, less than about 15 μm, about 10 μm to about 1000 μm, about 10 μm to about 100 μm, about 100 μm to about 250 μm, about 250 μm to about 750 μm, about 500 μm to about 1000 μm, about 250 μm to about 500 μm, about 500 μm to about 750 μm, about 750 μm to about 1000 μm, about 300 μm to about 400 μm, about 400 μm to about 500 μm, about 500 μm to about 600 μm, about 600 μm to about 700 μm, about 700 μm to about 800 μm, about 800 μm to about 900 μm, or about 900 μm to about 1000 μm. In some embodiments, the micro-channel may include a non-circular or non-semi-circular shape, and may include cross-sectional width of about 10 μm to about 50 μm.

FIG. 6A is an isometric view of a microfluidic device 600 formed according to one or more embodiments of the systems and methods described herein, and FIG. 6B is a cross-sectional view of the microfluidic device 600 of FIG. 6A. The microfluidic device 600 may include a micro gas chromatography device for use in a micro gas chromatography system or may include a micro heat exchanger. When used in a micro gas chromatography system, the microchannel may be coated with a coating, such as silica or other suitable for surface deactivation.

In some embodiments, the microfluidic device 600 includes a metallic structure having multiple surface regions including a first side surface region 601, a second side surface region 602 opposite to the first side surface region 601, a third side surface region 603 extending at least partially between the first side surface region 601 and the second side surface region 602, a fourth side surface region 604 opposite to the third side surface region 603 and extending at least partially between the first side surface region 601 and the second side surface region 602, a bottom surface region 606 extending at least partially between the first side surface region 601 and the second side surface region 602, and an upper surface region 605 extending at least partially between the first side surface region 601 and the second side surface region 602. While each of the surface regions 601-606 are shown generally flat or planar surfaces in FIG. 6A, in some embodiments, one or more (e.g. multiple or all) surface regions 601-606 may be non-planar (e.g., curved). The one or more of the multiple surface regions 601-606 define an inlet 621 and an outlet 622. While the inlet 621 is disposed on the first surface region 601 and the outlet 622 is disposed on the upper surface region 605 in the embodiment shown in FIG. 6A, the inlet 621 and the outlet 622 may be disposed on any multiple surface regions 601-606, including the same surface region.

Turning to FIG. 6B, the microfluidic device 600 includes a microchannel 620 disposed within the metallic structure and extending between the inlet 621 and the outlet 622, according to an embodiment. The microchannel 620 has a lateral width or diameter of about 1000 μm or less. For example, the microchannel 620 may have a maximum width of about 1000 μm or less, less than about 900 μm, less than about 800 μm, less than about 700 μm, less than about 600 μm, less than about 500 μm, less than about 450 μm, less than about 400 μm, less than about 350 μm, less than about 300 μm, less than about 250 μm, less than about 200 μm, less than about 150 μm, less than about 100 μm, less than about 75 μm, less than about 50 μm, less than about 25 μm, less than about 15 μm, about 10 μm to about 1000 μm, about 10 μm to about 100 μm, about 100 μm to about 250 μm, about 250 μm to about 750 μm, about 500 μm to about 1000 μm, about 250 μm to about 500 μm, about 500 μm to about 750 μm, about 750 μm to about 1000 μm, about 300 μm to about 400 μm, about 400 μm to about 500 μm, about 500 μm to about 600 μm, about 600 μm to about 700 μm, about 700 μm to about 800 μm, about 800 μm to about 900 μm, or about 900 μm to about 1000 μm. In some embodiments, the microchannel 620 may include a non-circular or non-semi-circular shape, and may include cross-sectional width of about 10 μm to about 50 μm.

The microchannel 620 may extend three-dimensionally within the metallic structure of the microfluidic device 600. For example, the microchannel 620 may extend (1) at least partially between the first side surface region 601 and the second side surface region 602, (2) at least partially between the third side surface region 603 and the fourth side surface region 604, and (3) at least partially between the upper surface region 606 and the bottom surface region 605. Said another way, in some embodiments, the microchannel 620 extends at least partially along a first theoretical plane or grid axis x, at least partially along a second theoretical plane or grid axis y angled relative to the first theoretical plane, and at least partially along a third theoretical plane or grid axis z angled relative to the first theoretical plane and the second theoretical plane.

The microfluidic device 600 may include any of the materials described herein, such as a stainless steel structure at least partially infiltrated with bronze. In some embodiments, the microfluidic device 600 includes a first internal region 670 defining at least a portion (e.g., all) of the microchannel 620. The first internal region 670 may be substantially free or devoid of pores (e.g., pores are absent from the region of the microfluidic device 600 defining the microchannel 620). The first internal region 670 may include bronze-infiltrated stainless steel. In some embodiments, the microfluidic device 600 also includes a second internal region 680 spaced from the first internal region and including one or more pores 690. Thus, any pores 690 present in the microfluidic device are spaced from the microchannel 620.

Example 1

In an example, plates including an elongated slot were 3D printed using stainless steel powder SS316L D90<22 μm from ExOne Co. and an ExOne Innovent+ binder jet printer. Each plate had a flat underside that acted as a top to the imprinted elongated slot in the plate below it. Excess printed powder was easily cleared from the plates using an air hose. Plates were stacked in an alternating pattern to form one continuous 610 μm diameter microchannel.

The base included an infiltrant holder and walls around the channeled portion (e.g., stacked plates) to hold the sacrificial powder. The sacrificial powder was SS316L 100 mesh (150 μm) from Alfa Aesar. The infiltrant was c90700 bronze powder (90% copper, 10% tin). A lid block raised the top of the stacked structure above the walls of the base, similar to the assembly shown in FIG. 4B. The stackable block structure was assembled, and a 180 g weight was placed on top.

The assembly was sintered at 800° C. for an hour and at 1135° C. for twenty minutes in an atmosphere of hydrogen and argon. During sintering, the weight visually reduced warping and improved the sealing between plates. Once sintered, bronze powder was poured into the infiltrant holder and sacrificial powder was poured around the stacked plates. The structure was infiltrated at 1135° C. for two hours in an atmosphere of hydrogen and argon.

The samples were milled to see the internal structure of the channels and inspect the sealing between plates and infiltrant overflow in the microchannels and the sacrificial powder. The infiltration further sealed the separate plates together, creating one long connected microchannel in the metallic structure. All portions of the microchannel remained clear of infiltrant. Most of the sacrificial powder filled, but large and small voids were present in the solidified sacrificial powder (see, for example, FIG. 6B).

This example indicated that it is possible to make long microchannels with small diameters out of stainless steel via binder jet 3D printing. An open-faced, segmented design allows powder to be cleared without sacrificing channel length. Sintering and infiltration seal the separate plates together. Infiltration can be controlled using sacrificial powder. This fabrication method could be used to create other complex geometries or to improve properties in 3D printed parts.

Example 2—Pore Filling

In order to determine parameters for sacrificial powder infiltration embodiments, a study was performed on bronze infiltration of sub-millimeter pores in binder jet printed stainless steel samples. Pore filling was characterized in microchannel segments with diameters of 370 μm, 650 μm, and 930 μm.

Printing: An ExOne Innovent+ binder jet printer was used to print stainless steel samples. The powder used was gas atomized spherical SS316L powder with D90<22 μm (ExOne Co.). ExOne aqueous binder (part #: 7100037CL) was used during printing and binder was cured by placing samples in an oven at 180° C. for at least 6 hours. After curing, samples were removed from the print bed and loose powder was removed from semi-enclosed features using brushes and compressed air at 25-55 PSI.

Sintering: Samples were sintered in a 2.7-inch diameter tube furnace that included a heating zone and a cooling zone (see supplementary material for a diagram of the furnace setup). For sintering, samples were placed into a stainless steel sample holder and pushed into the heating zone. Argon and hydrogen were then flowed through the tube at 700 SCCM and 200 SCCM, respectively, while the temperature was ramped from room temperature to about 800° C. (ramp rate about 13° C./min), held for 1 hour for binder burnout, ramped to the desired sintering temperature (ramp rate about 6° C./min), and held for 20 min. Samples were cooled rapidly as follows. For the last 5 min of the sintering time, hydrogen flow was stopped, and argon flow was increased to 1400 SCCM to flush the hydrogen from the tube. After this, the sample was rapidly moved to the cooling zone (˜200° C.), left there for 20 minutes still under the argon flow, and then removed from the furnace.

Infiltration: After sintering, samples for the pore filling study were infiltrated with bronze using −325 mesh 10% Sn bronze powder (OzoMetal LLC) as follows. The needed amount of bronze infiltrant depended on the post-sintering void fraction of the porous printed matrix and was calculated according to the equation:

$M_{bronze} = {\left( {\frac{M*V_{design}*\varepsilon_{V}}{M_{avg}} - \frac{M}{\rho_{SS}} + {V_{channels}*\varepsilon_{V}}} \right)*\beta*f*\rho_{bronze}}$

The terms in the equation are as follows: M is the post-sintering measured mass of the individual sample, M_(avg) is the average post-sintering mass of samples, ε_(V) is the fractional volumetric shrinkage due to sintering (calculated by measuring part dimensions before and after sintering), ρ_(SS) is the density of bulk SS316L, V_(channels) is the total designed volume of the channels, β is the volumetric thermal expansion of stainless steel at 1000° C. which affects the void volume of the part (about 1.05), f is the desired fractional fill of the total void volume, and ρ_(bronze) is the estimated density of liquid bronze at 1000° C. (about 7.85 g/cm³). The bronze was placed into an infiltrant holder printed as an integral part of the sample. Samples were placed into a quartz lined stainless steel sample holder and put into the furnace by initially setting the sample inside the cooling zone, closing the end cap, and flowing argon at 1400 SCCM for 5 minutes to flush air from the tube. The sample was then moved to the heating zone (about 800° C.), argon and hydrogen flow were set to 700 SCCM and 200 SCCM, respectively, and the furnace was ramped to the infiltration temperature where it was held for the desired infiltration time (ramp rate about 6° C./minute). Samples were then cooled according to the same rapid cooling procedure used after sintering. After removal from the furnace, the sample was optically imaged (while backlit with a white LCD screen) to determine which holes were empty and fill height was measured as the distance from the base of the sample to the highest filled hole in each column.

Results: To determine the dependence of pore filling on pore size, samples with three different pore sizes were prepared and results were compared to capillary model predictions. Each sample was designed with through holes (horizontal microchannel segments) allowing easy observation of pore filling. These through-hole samples were printed, sintered, and infiltrated. Various infiltration conditions (shown in Table 1) were used to explore the effect of infiltration time and temperature on pore filling. Enough bronze powder was added to the bronze holder to allow filling of the porous printed matrix and roughly half of the through holes to allow comparison of the fill height in different hole sizes.

TABLE 1 Infiltration Times and Temperatures Sintering Infiltration Condition 1 20 min at 1085 C. 3.5 hrs at 1085 C. Condition 2 20 min at 1085 C. 5 hrs at 1085 C. Condition 3 20 min at 1135 C. 3 hrs at 1135 C. Condition 4 20 min at 1135 C. 5 hrs at 1135 C.

Filled holes were readily observed due to the blocked light transmission. The term fill height is used to denote the highest filled hole in each vertical column of holes. There are several observations from these results:

-   -   1. The fill height of smaller holes was generally greater than         that of larger holes.     -   2. In same-sized holes, there was significant variation in fill         height across the sample. This lateral variation is well         visualized in the medium holes where fill height can go up and         down gradually across the sample.     -   3. There are abrupt fill height changes between adjacent         vertical columns of different sized holes. These abrupt changes         are very different than the gradual changes in same sized holes         mentioned above.     -   4. In almost all columns, holes are filled from the bottom up         with no empty holes below the highest filled holes. Across the         12 samples, the small hole columns almost always completely         filled to the top of the sample (completely filled in 99 of 108         columns).         Observations 1, 3, and 4 are qualitative and in good agreement         with capillary model predictions. The variation described in         observation 2 represents a significant deviation from the model.

To analyze the variation in fill height of same-sized holes, histograms of fill heights of medium and large round hole columns were prepared. Because each sample had a slightly different infiltrant fill level, to allow for comparison between samples, fill height data in each sample were adjusted using the following equation:

h _(adj) =h−( h _(sample) −h _(all))

where h_(adj) is the adjusted fill height data point, h is the unadjusted fill height data point, h _(sample) is the mean fill height of medium holes in the sample, and h _(all) is the mean fill height of medium holes across all samples. This made the mean fill height of data points in each sample the same as all other samples.

Medium hole columns showed fill-height distributions which have standard deviations and standard errors of 2.4 mm and 0.17 mm respectively. Large hole columns showed fill-height distributions which have standard deviations and standard errors of 4.5 mm and 0.70 mm respectively. Empty small hole data was too sparse in these samples for statistical analysis. There was no clear dependence of pore filling on infiltration time or temperature, therefore all four infiltration conditions were lumped together for this statistical analysis. In 5 of the 12 samples, either the medium holes were completely filled or the large holes were completely empty; these cases did not allow for analysis of variation in fill heights and therefore were not included in the statistical analysis.

Based on the fill-height data, two additional quantitative findings of note are as follows: 1) the average difference in fill height between medium and large hole columns is 8.3 mm and 2) the difference between the predicted fill volume (based on calculated void volume and infiltrant mass) and the measured fill level was found to be ±10%.

Example 3—Sacrificial Powder Infiltration Proof-of-Concept

After the pore filling study of Example 2, the parameters obtained from Example 2 were used to design and implement a proof-of-concept for the sacrificial powder infiltration process by fabricating 700 μm- and 930 μm-diameter microchannels, each 37 mm long.

Printing in Example 3 followed the same parameters as Example 2. In Example, sonication was also used for powder removal during which a small wire was inserted into the channels to loosen and remove unbound powder.

Applying the Capillary Model of Infiltration in Example 3: Adjustments to the capillary model of infiltration made for Example 3 after Example 2 included: 1) a standard deviation in infiltrant pressure of 180 Pa was included to account for the variation in effective pressure, 2) capillary pressures for all pore sizes were scaled down by a factor of 3 to account for surface tension uncertainty or other factors that could affect capillary pressure, and 3) an uncertainty in total void volume of ±10% was included. For high yield fabrication using Example 3, the microchannels, the sacrificial powder, and the porous printed matrix must have capillary pressures that are different enough to account for these model adjustments.

Infiltration: Samples were infiltrated using the same process as Example 2 with a few differences. Prior to infiltration, sacrificial powder was poured into the sacrificial powder cavities (see, for example, FIG. 2C). Sacrificial powder was water atomized −100 mesh (particle diameter <150 μm) SS316L powder (Thermo Fisher Scientific, product #: 11089). Sacrificial powder mass and volume were measured to calculate a void fraction of about 58%. Samples were placed into an alumina crucible and bronze powder was placed in the crucible around the sample. Following infiltration, channel filling was checked using backlighting and samples were milled for optical imaging.

Results: Based on the adjustments described above, an experiment was performed to test sacrificial powder and bronze infiltrant fabrication of microchannels. Three samples were designed as shown in FIG. 3A, printed, sintered (20 min at 1135° C.), and infiltrated (3 hours at 1135° C.). Samples were infiltrated with enough bronze to both fill the porous printed matrix and partially fill the sacrificial powder reservoirs. Capillary pressures for the porous printed matrix, the sacrificial powder, and the channel were calculated using equations above and are shown in Table 2. To ensure high-yield fabrication, the particle diameter of the sacrificial powder was chosen so that the differences in calculated capillary pressures between the three pore types in Table 2 were much larger than the variation in effective infiltrant pressure, even accounting for the scaling down of capillary pressures. The void volume of the sacrificial powder reservoirs was chosen to be approximately 40% of the total void volume to manage void volume uncertainty. Additionally, sacrificial powder was placed in close proximity with the channels to reduce the impact of infiltrant pressure variation across the part.

TABLE 2 Theoretical Capillary Pressures Equivalent P_(c) Pore P_(c) (Scaled D_(sv) Diameter (Unscaled) down by 3) Porous 8.7 μm ~3 μm −1200 kPa (Eq. 3) −400 kPa Printed Matrix Sacrificial 70 μm ~60 μm −62 kPa (Eq. 3) −20 kPa Powder 700 μm N/A 700 μm −5.7 kPa (Eq. 2) −1.9 kPa Channel

Calculations for Table 2 were performed as follows. The capillary pressure Pc of the channel was calculated with a channel diameter of 700 μm. Pc for the sacrificial powder and the porous printed matrix was calculated with porosities ε of 0.58 and 0.36, respectively, and mean surface volume particle diameters D_(sv) of 70 μm and 8.3 μm, respectively, where D_(sv) was determined by assuming spherical powder with normal distributions (having standard deviation ⅓ the average diameter) around averages of 80 μm and 10 μm for the sacrificial powder and the porous printed matrix, respectively. Equivalent pore diameters for the powders were calculated from their values of P_(c).

Post infiltration analysis showed that in each sample: 1) the porous printed matrix was filled, 2) the sacrificial powder reservoir was partially filled, and 3) none of the 700 or 930 μm channels were filled. The optical image in FIG. 3 is from a region of a milled, infiltrated sample and shows the well filled printed matrix 270, the partially filled sacrificial powder reservoirs 280 and pores 290, and an unfilled 700 μm channel 220.

In Example 3, twenty microchannels were successfully fabricated in each of three samples. These results show that sacrificial powder and bronze infiltration can be used for high-yield fabrication of metal microchannels, as predicted by the large differences between capillary pressures for each of the different pore types. Additionally, fabrication was very likely successful because the sacrificial powder pores accounted for a large fraction of the total void volume and because the sacrificial powder was in close proximity to the channels. This work shows that with a few adjustments, the capillary model of infiltration is a useful tool for microfluidic fabrication design.

It may be concluded that when using SPI for fabrication of parts with internal structures of different sizes than the channels fabricated here, the size of the sacrificial powder may be adjusted to ensure a large gap in capillary pressure is maintained between the different pore types. Additionally, if desired, sacrificial powder may be cut off the part after infiltration.

Examples 2 and 3 advance additive manufacturing of metallic microfluidic devices by contributing sacrificial powder infiltration, a new method for infiltrant pressure control, and using this method to form sealed metallic microchannels. Sacrificial powder infiltration (including bronze) was used to seal the porous printed matrix of binder-jet printed parts while keeping printed microchannels free of infiltrant. Pore filling in capillary segments with various diameters were analyzed and it was found that while pore filling generally followed predictions from a capillary model of infiltration, there were significant deviations from the predicted behavior. Channels of a certain size did not always fill to the same height and the difference in fill height between channels of different sizes was much smaller than predicted. Based on these observations, adjustments were made when using the capillary model of infiltration to guide microfluidic device design. Sacrificial powder infiltration (including bronze) to fabricate 700 μm and 930 μm metallic microchannels. Cross-sections of the parts show fully dense printed matrix, semi-porous sacrificial powder, and empty microchannels. The large difference in capillary pressure for these three structures enabled high-yield fabrication.

As used herein, the term “about” or “substantially” refers to an allowable variance of the term modified by “about” by ±10% or ±5%. Further, the terms “less than,” “or less,” “greater than”, “more than,” or “or more” include as an endpoint, the value that is modified by the terms “less than,” “or less,” “greater than,” “more than,” or “or more.”

While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiment disclosed herein are for purposes of illustration and are not intended to be limiting. 

What is claimed is:
 1. A method for fabricating a microchannel in metal, the method comprising: providing a first metallic plate having a first surface with an elongated slot recessed therein; providing a second metallic plate having a second surface; interfacing the first surface of the first metallic plate with the second surface of the second metallic plate with the second surface effective to cover the elongated slot to form a microchannel between the first metallic plate and the second metallic plate; and thermal bonding the first metallic plate to the second metallic plate to form a metallic body having the microchannel extending therethrough.
 2. The method of claim 1, further comprising infiltrating the metallic body with an infiltrant.
 3. The method of claim 2, wherein infiltrating the metallic body with an infiltrant includes infiltrating the metallic body with the infiltrant in the presence of a sacrificial powder.
 4. The method of claim 3, wherein the infiltrant includes a bronze powder and the sacrificial powder includes a stainless steel powder.
 5. The method of claim 3, wherein infiltrating the metallic body with the infiltrant in the presence of a sacrificial powder includes infiltrating the metallic body with the infiltrant in the presence of a sacrificial powder at a predetermined time, at a predetermined pressure of about −1 kPa or less, and at a predetermined temperature of at least 1000° C. to melt the infiltrant.
 6. The method of claim 5, wherein infiltrating the metallic body with the infiltrant in the presence of a sacrificial powder forms a structure including: a first region including at least a portion of the metallic body and defining the microchannel, the first region being substantially devoid of pores; and a second region including at least a portion of the solidified sacrificial powder and spaced from the microchannel by the first region, the second region including one or more pores.
 7. The method of claim 5, wherein infiltrating the metallic body with the infiltrant in the presence of a sacrificial powder at a predetermined time and a predetermined temperature of at least 1000° C. to melt the infiltrant includes infiltrating the metallic body with the infiltrant in the presence of the sacrificial powder at a predetermined time of at least one hour and the predetermined temperature of at least 1000° C. in an atmosphere of hydrogen and argon to melt the infiltrant.
 8. The method of claim 3, further comprising, before infiltrating the metallic body with the infiltrant in the presence of a sacrificial powder, disposing the sacrificial powder adjacent the metallic body.
 9. The method of claim 2, wherein thermal bonding the first metallic plate to the second metallic plate to form a metallic body having the microchannel extending therethrough includes sintering the first metallic plate to the second metallic plate at a predetermined temperature of at least 700° C. for a predetermined time of at least one hour in an atmosphere of hydrogen and argon to form the metallic body having the microchannel extending therethrough.
 10. The method of claim 1, wherein the microchannel has a width of less than about 1000 μm.
 11. The method of claim 1, wherein thermal bonding the first metallic plate to the second metallic plate includes sintering the first metallic plate to the second metallic plate at a predetermined temperature of at least 1250° C., and infiltration is absent from the method.
 12. The method of claim 1, further comprising disposing a weight on at least one of the first metallic plate or the second metallic plate before thermal bonding the first metallic plate to the second metallic plate.
 13. The method of claim 1, further comprising three-dimensional printing the first metallic plate and the second metallic plate with a stainless steel material.
 14. The method of claim 1, further comprising: disposing a metallic lid plate over the first metallic plate and the second metallic plate with the second metallic plate being positioned between the first metallic plate and the second metallic plate, wherein when the first surface of the first metallic plate is interfacing with the second surface of the second metallic plate with the second surface and the metallic lid plate is disposed over the first metallic plate and the second metallic plate an opening on the metallic lid plate is in fluid communication with the elongated slot in the first surface of the first metallic plate via at least an inlet in the second metallic plate positioned proximate to a terminating end region of the elongated slot; wherein thermal bonding the first metallic plate to the second plate includes sintering the first metallic plate, the second metallic plate, and the metallic lid plate together to form the metallic body having the microchannel extending therethrough with the opening on the metallic lid plate in fluid communication with the microchannel.
 15. The method of claim 14, further comprising: disposing an additional metallic plate over the first metallic plate and the second metallic plate such that the additional metallic plate is between the second metallic plate and the metallic lid plate, the additional metallic plate including an additional elongated slot, wherein when the first surface of the first metallic plate is interfacing with the second surface of the second metallic plate with the second surface and the metallic lid plate is disposed over the first metallic plate and the second metallic plate with the additional metallic plate positioned between the metallic lid plate and the second metallic plate, the additional elongated slot is in fluid communication with the opening in the metallic lid plate and the elongated slot of the first metallic plate; wherein sintering the first metallic plate to the second plate includes sintering the first metallic plate, the second metallic plate, the additional metallic plate, and the metallic lid plate together to form the metallic body having the microchannel including at least the elongated slot and the additional elongated slot extending therethrough with the opening on the metallic lid plate in fluid communication with the microchannel.
 16. The method of claim 15, wherein the microchannel extends at least partially between a first side surface region of the metallic body and a second side surface region of the metallic body opposite to the first side surface region, extends at least partially between a third side surface region of the metallic body extending at least partially between the first side surface region and the second side surface region and a fourth side surface region opposite to the third side surface region, and extends at least partially between an upper surface region of the metallic body and a bottom surface region of the metallic body opposite to the upper surface region.
 17. A microfluidic device, comprising: a plurality of plates that are thermally bonded together to form a body having an inlet and an outlet, pores of the plurality of plates being at least partially filled with an infiltrant; a microchannel extending three-dimensionally through the within the metallic structure between the inlet and the outlet, wherein the microchannel has a lateral width or diameter of about 1000 μm or less.
 18. The microfluidic device of claim 15, wherein the metallic structure includes: a first internal region defining at least a portion of the microchannel and substantially devoid of pores; and a second internal region spaced from the first internal region and including one or more pores.
 19. The microfluidic device of claim 16, wherein the metallic structure is stainless steel and the first region includes bronze-infiltrated stainless steel
 20. The microfluidic device of claim 15, wherein the microfluidic device includes a micro gas chromatography device.
 21. The microfluidic device of claim 15, wherein the microfluidic device includes a micro heat exchanger.
 22. A microfluidic device, comprising: a stainless steel metallic structure having an inlet and an outlet; and a microchannel disposed within the metallic structure and extending between the inlet and the outlet, wherein the microchannel has a lateral width or diameter of about 1000 μm or less and extends within the metallic structure at least partially along a first theoretical plane, at least partially along a second theoretical plane angled relative to the first theoretical plane, and at least partially along a third theoretical plane angled relative to the first theoretical plane and the second theoretical plane. 